Spinal implants of the above-mentioned type are used in pairs and in combination with a connecting rod to treat arthrosis and vertebral fractures or to correct distortions of the vertebral column such as scoliosis or kyphosis.
To immobilize two vertebrae, a surgeon uses four spinal implants, two on each vertebra. To this end the surgeon engages the anchoring parts of the implants in the vertebrae and then fastens the implants of the two pairs together by means of two connecting rods, each of which extends from one vertebra to the other between the two implants. Those two connecting rods are bent beforehand as a function of the correction to be applied to the vertebral column. In that way the two rods are positioned parallel to the vertebral column to hold it substantially straight.
Two types of spinal implant are more particularly known in the art. Single-axis implants in which the anchoring part and the mounting part are aligned are known. In the majority of those implants, the anchoring part and the mounting part are stationary relative to each other and are formed as a single piece. Some of those single-axis implants are nevertheless such that their anchoring part and their mounting part are connected to each other by a pivot connection about the longitudinal axis of the implant.
Also known are multi-axis implants in which the anchoring part and the mounting part are connected to each other by a ball-joint connection.
Here the expression “ball-joint connection” refers to a connection that allows the mounting part to assume different inclinations relative to the axis of the anchoring part. The anchoring part and the mounting part are thus stationary in translation relative to each other along three axes in space but free to rotate about each of the three axes.
The expression “pivot connection” refers to a connection that allows the mounting part to turn about the anchoring part, remaining in alignment with its axis. The anchoring part and the mounting part are thus stationary in translation relative to each other along the three axes in space, stationary in rotation relative to each other about two of the three axes, and free to rotate about the axis of the anchoring part.
The surgeon may be called on to use one or the other of those two types of spinal implant, for example as a function of the trauma to be treated and the shape of the vertebral column of the patient.
There is a drawback in that the implant manufacturer must double the number of product lines to offer both types of spinal implant that surgeons need to operate on patients.
Surgeons for their part must manage stocks of the two types of implant and, before each operation, predict which type of spinal implant they are going to use.
One example of a multi-axis implant is described in Document EP 1 839 606. As described in that document, the implant includes:                a screw having a head that is part-spherical;        a tubular receiving part including at its bottom end an opening to receive the head of the screw;        a tubular pressure member engaged inside the receiving part so that its bottom end bears against the head of the screw, and its top end forms a cradle for receiving a connecting rod; and        a closure device comprising both an outer bolt screwed into the top end of the tubular receiving part in order to lock the pressure member against the head of the screw so as to prevent it from moving, and also an inner bolt screwed into the outer bolt to lock the connecting rod at the bottom of the cradle provided in the pressure member.        
In the above document, the means for locking the ball-joint connection provided between the head of the screw and the receiving opening of the tubular receiving part are therefore situated above the receiving cradle of the connecting rod.
The drawback of the above implant is that the ball-joint connection can be locked only after the connecting rod has been engaged in the implant. Thus the surgeon is not able to lock it before operating on the patient.
There is also known from Document US 2007/0288004 a spinal implant including:                a screw including a part-spherical head;        a tubular tulip, defining internally an axial housing for receiving transversely a connecting rod and that has a threaded top part and a spherical bottom part;        a rosette that is adapted to be housed in the spherical bottom part of the tulip;        a locking member that is adapted to be screwed into the thread of the top part of the tulip to lock the connecting rod against the rosette, the effect of which is to compress the rosette in the spherical bottom part of the tulip and thus to prevent the screw from moving relative to the tulip.        
The rosette has a stud adapted to be engaged in the socket in the head of the screw so that once the tulip is assembled to the screw the implant behaves as a single-axis implant. This rosette may moreover be replaced by a rosette with no stud so that the implant may behave as a multi-axis implant.
However, in the above document, the means enabling the head to pivot relative to the tulip and to lock the multi-axis function are carried by a single part, the rosette. Because of this, to change from a single-axis implant to a multi-axis implant, it is necessary to remove the whole of the rosette and therefore to dismantle the implant. Thus the surgeon is not able to lock the multi-axis function while operating on the patient.